1. Field of the Invention
The present invention relates to a display apparatus for displaying an image by driving a current to flow to a light emitting device placed in each pixel. To put it in more detail, the present invention relates to an image display apparatus pertaining to the so-called active-matrix type category as a display apparatus employing an insulated-gate type field-effect transistor in every pixel as a transistor for controlling the magnitude of a current flowing through a light emitting device included in the pixel as a device typically made of an organic EL device.
2. Description of the Related Art
In an image display apparatus such as a liquid-crystal display apparatus, a large number of liquid-crystal pixels are laid out to form a matrix. An image is displayed by controlling the transmittance and reflectance of a light beam arriving at each pixel in accordance with information on the image to be displayed. This principle also holds true of other display apparatus such as an organic EL display apparatus employing an organic display device in each pixel. Unlike the liquid-crystal pixel, however, an organic EL device is a self-light-emitting device requiring no backlight. Thus, the organic EL display apparatus has excellent visibility in comparison with the liquid-crystal display apparatus. Other merits offered by the organic EL display apparatus include a high response speed. In addition, the luminance level (or the gradation) can be controlled in accordance with the magnitude of a current flowing through the organic EL display device by execution of the so-called current control, which is much different from the voltage control of the liquid-crystal display device.
Much like the liquid-crystal display apparatus, the organic EL display apparatus may adopt a simple matrix method or an active matrix method as the driving method. The simple matrix method requires a simple configuration of the image display apparatus, but has a problem that an image display apparatus having a large size and a high degree of fineness is difficult to implement. Thus, at the present time, the active matrix method is being developed by a large number of manufacturers of image display apparatus. In accordance with this active matrix method, the current flowing through a light emitting device employed in every pixel circuit is controlled by using an active device also included in the pixel circuit. In general, the active device is a TFT (a thin film transistor). The active matrix method is described in the following documents: Japanese Patent Laid-open No. 2003-255856 (Patent Document 1); Japanese Patent Laid-open No. 2003-271095 (Patent Document 2); Japanese Patent Laid-open No. 2004-133240 (Patent Document 3); Japanese Patent Laid-open No. 2004-029791 (Patent Document 4); Japanese Patent Laid-open No. 2004-093682 (Patent Document 5).